For some applications the ability to provide foreground/background separation in an image is useful. In PCT Application No. PCT/EP2006/005109 separation based on an analysis of a flash and non-flash version of an image is discussed. However, there are situations where flash and non-flash versions of an image may not provide sufficient discrimination, e.g. in bright sunlight.
Depth from de-focus is a well-known image processing technique which creates a depth map from two or more images with different focal lengths. A summary of this technique can be found at: http://homepages.inf.ed.ac.uk/rbf/CVonline/LOCAL_COPIES/FAVARO1/dfdtutorial.html. Favaro is based on a statistical analysis of radiance of two or more images—each out of focus—to determine depth of features in an image. Favaro is based on knowing that blurring of a pixel corresponds with a given Gaussian convolution kernel and so applying an inverse convolution indicates the extent of defocus of a pixel and this in turn can be used to construct a depth map. Favaro requires a dedicated approach to depth calculation once images have been acquired in that a separate radiance map must be created for each image used in depth calculations. This represents a substantial additional processing overhead compared to the existing image acquisition process.
US 2003/0052991, Hewlett-Packard, discloses for each of a series of images taken at different focus distances, building a contrast map for each pixel based on a product of the difference in pixel brightness surrounding a pixel. The greater the product of brightness differences, the more likely a pixel is considered to be in focus. The image with the greatest contrast levels for a pixel is taken to indicate the distance of the pixel from the viewfinder. This enables the camera to build a depth map for a scene. The camera application then implements a simulated fill flash based on the distance information. Here, the contrast map needs to be built especially and again represents a substantial additional processing overhead over the existing image acquisition process.
US 2004/0076335, Epson, describes a method for low depth of field image segmentation. Epson is based on knowing that sharply focussed regions contain high frequency components. US 2003/0219172, Philips, discloses calculating the sharpness of a single image according to the Kurtosis (shape of distribution) of its Discrete Cosine Transform (DCT) coefficients. US 2004/0120598, Xiao-Fan Feng, also discloses using the DCT blocks of a single image to detect blur within the image. Each of Epson, Philips and Feng is based on analysis of a single image and cannot reliably distinguish between foreground and background regions of an image.
Other prior art includes US 2003/0091225 which describes creating a depth map from two “stereo” images.
It is an object of the invention to provide an improved method of distinguishing between foreground and background regions of a digital image.